The transmission of information along a transmission path involves a transmission medium. Any change in impedance that a transmission medium presents to the transmission path will result in energy being reflected or dispersed in some way. An ideal transmission path will deliver all of its signal energy to a destination without any internal loss or corruption of that signal energy. One of the most efficient media for this task is coaxial cable. Unshielded twisted pair (UTP) is often used in place of coaxial cable to reduce costs.
In digital data transmission, small imperfections along the length of a transmission line cable can result in differential to common mode conversion of signal energy. This results in radiation of energy from the cable in the form of electromagnetic emission. Conversely, any common mode energy picked up by the cable will leak into the system due to common to differential mode conversion caused by the same imperfections in the cable. Energy loss in the transmission line cable is problematic at particular frequencies related to the fundamental and harmonic components of a transmitted signal. Energy pick up is exacerbated at frequencies which relate to the length of the transmission line. If an applied energy source is swept across a range of frequencies, energy radiates from the cable with a high efficiency at many discreet frequencies and their respective harmonics. That efficiency depends upon the fractional relationship between the length of the transmission line and the wavelength of the applied signal source. At other frequencies where the wavelength of the signal is odd multiples of .lambda./8, the radiation from the line drops to a low value. The improved electromagnetic interference performance for that transmission line is due to the fact that equal energy is contained in the electric field and magnetic field of the transmission line so the transmission line behaves as though it were terminated by an impedance equal to its "characteristic" impedance. This "terminated" condition is analogous to an infinite transmission line. If a transmission line is not correctly terminated, energy is reflected up and down the line to produce voltage and current "standing waves" on that line. As the energy passes certain points in the line it adds to either the magnetic or electric field, intensifying the electromagnetic field being radiated from that point. Similarly, the addition of further energy to the line from an external electro-magnetic field reinforces any energy already being reflected up and down the line due to earlier field pick-up. Accordingly, an increased sensitivity to external fields will occur along the transmission line at points which correlate to the standing wave patterns.
A particular problem occurs in signal lines or cables comprising multiple pairs of transmission lines such as those found in four pair UTP, typically used in token ring and telephony distribution networks in which not only is there coupling between the lines in one pair but also between pairs of lines.
Twisted pair is one of the most common transmission media used in communication networks and consists of two insulated copper wires, typically 1 mm thick, twisted together in the form of a helix. In larger communication networks, several pairs of twisted pairs are bundled together to form a single cable. IEEE Standard 802.5, the well known token ring network standard, is often implemented using twisted pair cable in which each twisted pair is electrically shielded to reduce the effects of electromagnetic emissions and interference. This shielding makes this type of twisted pair cable relatively expensive.
It is currently proposed to increase the bit rate of the local area network systems to a speed of up to 622 Mbit s.sup.-1 and preferably use low grade unshielded twisted pair cable, being less expensive, for new installations and allow usage in existing installations. The unshielded twisted pair cable and higher transmission rate introduces electromagnetic emission and susceptibility problems which have to be overcome to meet International compatibility standards and to ensure the integrity of transmitted data.
The twisted pair, as a high frequency signal transmission line, has a characteristic impedance which, as noted above, must be matched to that of a load to which it is connected to ensure that the maximum signal power is transferred. If the characteristic impedance of the transmission line is not matched to the load then the transmitted signal received at the load will be reflected back along the transmission line which can then be emitted as electromagnetic signals along its length. The reflected signal will also tend to corrupt the incoming signal. If the reflected signal reaches the other end of the transmission line the signal will interfere with the source of the transmitted signal.
Another feature of a twisted pair is that it is susceptible to electromagnetic interference from many sources such as nearby power cables or even an adjacent twisted pair within a cable bundle. This electrical "pick-up" can appear as a common mode unwanted signal on each wire of the twisted pair transmission line, with respect to a distant reference point.
One known technique of rejecting common mode noise is the use of a pair of differential transmitter and receiver circuits. In this arrangement a differential transmitter circuit at one end of the transmission line produces two signals of equal and opposite polarity for every binary 1 or 0 signal to be transmitted and so transmits opposite polarity signals along the two wires of the twisted pair. The differential receiver, located at the other end of the transmission line, is only sensitive to the difference between the two inputs from the twisted pair so that any common mode energy picked up on both wires is rejected. In practice, even this technique is not wholly reliable due to imperfections in the helical winding of the twisted pair and the mutual inductance of the twisted pair. Also, it is practically impossible to balance the signals in the two wires of the twisted pair. Accordingly, it is often necessary to use shielded twisted pair cable to minimize emissions and electrical interference when transmitting digital data.
Common mode chokes are also typically used in termination circuits for transmission lines to reject unwanted common mode signals. In one application of a conventional common mode choke, a communications cable, such as twisted pair, carrying a differential signal, is wound around a ferrite toroid to increase the series inductance of the whole cable, thus raising the impedance to unwanted high frequency common mode signals. The model of a common mode choke is a series connected transformer. The transformer action ensures that the differential signal on the two wires appears at both ends of the device. A potential reduction of some 15 to 20 dB of common mode energy, with an insertion lost of some 0.5 dB for differential mode energy, can be obtained by the correct application of a common mode choke.
FIG. 10 shows the transformer model of an ideal common mode choke 28 as applied to a differential transmission line 29 with a double ended load R.sub.1, R.sub.2. In this ideal device, if a high frequency current source i.sub.1 is applied to one of the two inputs of the common mode choke 28, it forces a current through the upper winding 30 of the common mode choke and across the load resistor R.sub.1, developing a voltage v.sub.1. A similar current -i.sub.1 is induced in the lower winding 31 of the common mode choke, so developing a negative voltage -v.sub.1 across load resistor R.sub.2. A high frequency current source i.sub.2 applied to the other input develops a voltage v.sub.2 across load R.sub.2 and induces a negative voltage -v.sub.2 across the load resistor R.sub.1.
Accordingly, if i.sub.1 =i.sub.2 then v.sub.1 =v.sub.2 and providing R.sub.1 =R.sub.2 then all the currents and voltages cancel so that the common mode impedance looking into the input side of the common mode choke appears infinite for all frequencies. However, if i.sub.1 and i.sub.2 are not equal, as is the case for differential data signals, then the difference in current develops an equal voltage across both load resistors and therefore the common mode choke appears as a short circuit between the current source and load.
The performance of a practical common mode choke is in fact limited by the parasitic elements of the device. Common mode rejection is dominated by the core characteristics and at higher frequencies the permeability and inductance of the core decreases so that the impedance does not appear infinite for all frequencies. Lower frequency common mode signals carried by the windings contribute significantly to the magnetic flux of the core causing the core to saturate at relatively low levels of common mode energy. In practice, a current shunt path to ground is provided to partially compensate for this but the maximum available rejection is still rather limited.
In the present application, the term "differential transmission line" is to be understood to mean a double ended signal line in which each signal path has a differential data signal imposed upon it. Accordingly, the present invention is not limited to the field of communication systems but also has applications in high frequency apparatus, and in particular electrical consumer goods. Examples of suitable transmission lines are shielded twisted pair, unshielded twisted pair, flat ribbon cable, twin core cable, PCB microstrip transmission lines and pairs of conventional co-axial cables.